1. An idiot's laugh
2. An alternate version of "duh" for use on really fucking stupid people.
3. A greeting
4. Something to say when there's nothing else to say.
5. An exclamation which basic meaning is "oh how obvious!"

1.
You: <tells a joke>
Idiot: hurr hurr hurr!

2.
Idiot: Wow I didn't know post it notes came in other colors!
You: Hurr.